1. Field
This invention relates generally to touch sense interface circuits, and more specifically a touch sense interface circuit that can be used under noisy electromagnetic conditions.
2. Related Art
A touch sense interface circuit, when coupled to a touch panel, recognizes occurrence of a touch when a user's finger is touching or is very close to, such as a few millimeters from, a touch pad of the touch panel.
A capacitive touch sense interface circuit usually provides good results under normal conditions but its operation may be disrupted when subjected to electromagnetic (EM) aggression. EM aggression occurs when EM noise emanating from outside of a circuit, or external EM noise, interferes with operation of the circuit.
The magnitude, frequency and duration of the external EM noise can vary in an unknown manner. Disruption of operation of a capacitive touch sense interface circuit due to external EM noise is more likely to occur in a low-power capacitive touch sense interface circuit because the power of the external EM noise can be high relative to the power of signals used for operation of the low-power capacitive touch sense interface circuit.
Many capacitive touch sense interface circuits compare an internal time reference with external timing of charge or oscillation that is dependent on presence or absence of a touch. One known capacitive touch sense interface circuit determines a ratio of internal oscillation frequency with an external oscillation frequency. A touch causes a capacitance change, which changes said ratio, and, as a result, the touch is detected.